1. Field of the Invention
The invention relates generally to methods for forming Extreme Ultraviolet (EUV) Lithography optical elements such as EUV mirrors substrates and EUV photomask substrates. More specifically, the invention relates to methods for forming high quality lithography glass structures by extrusion and consolidation of a pre-form to provide a thermally stable ultra low expansion glass lithography substrate.
2. Background
Ultra low expansion glass, such as a type sold by Coming Incorporated, Corning, N.Y. under the trade name ULE® glass is formed by a process known as flame hydrolysis. Chemical vapors are introduced into a gas-oxygen burner at approximately 1700° C. The combustion reaction forms sub-micron sized molten particles. The burners are generally aligned over a rotating table in a refractory furnace where the particles are collected and fused into a large, dense, solid boule of glass. Typical materials used to make such glass includes high-purity titanium-containing feedstock, such as titanium alkoxide, titanium tetrachloride or titanium isopropoxide and high purity silicon-containing feedstock such as octamethylcyclotetrasiloxane or silicon tetrachloride. During the combustion process, titanium doped silica soot is formed. Much of the soot is deposited on the rotating table, but some of the soot becomes a waste product of the silica forming process. The soot typically has a very high purity and very small particle size. Much of the soot is deposited on the rotating table, but some of the soot is not deposited and becomes an exhausted waste product of the silica forming process which is collected in a baghouse. The soot typically has a very high purity and very small particle size.